The present invention relates generally to processes for preparing mixed oxide powders such as those used in the ceramics industry, and in particular to polymerized organic-inorganic routes to such mixed oxide powders.
As further background, mixed oxide powders have been prepared in the past by a number of synthetic routes, including for example traditional high temperature solid-state routes using metal oxides or carbonates, and chemical routes. As the requirements of purity, phase distribution and other physical characteristics of ceramic powders become more stringent, chemical synthesis methods are becoming more popular. Chemically prepared powders are pure and homogeneous at a molecular level [1]. Their chemical and physical characteristics can be closely controlled. They are synthesized at significantly lower temperatures than conventional ceramic powders. In addition, the costly grinding-refiring steps are eliminated.
One such chemical synthesis route is the solution-polymerization technique, better known as the xe2x80x9cPechini Methodxe2x80x9d [2, 3]. The technique employs the Pechini resin as the polymeric carrier of the pre-ceramic powders. The resin consists of citric acid, as the chelating agent, and ethylene glycol, to promote polymerization during the esterification process. The cation sources are usually nitrate salts of the metals, mixed in stoichiometric proportions in the aqueous resin solution. Modifications have been suggested to move from costly nitrate salts to more readily available sources, specifically for silicon containing systems [4]. An important aspect in the selection of these cation precursor salts is the aqueous solubility. Alternatives have been suggested to use other resins in order to reduce the cost [5, 6].
In the literature [6-10], several basic reactions are proposed to occur during the formation of the Pechini precursors. The heart of the process is chelation of the dissolved metal ions by the carboxylic acid end of the citric acid which has a configuration ideal for chelating. Polyesterification follows as the temperature of mixture is increased. The chelating action of the organics is suggested to be responsible for the formation of a stable, atomistically homogeneous, pre-ceramic organometallic.
In light of this background, there remains a need for a chemical route to mixed oxide powders which produces powders of high purity at relatively low temperatures and high yields, utilizing readily available and relatively inexpensive starting materials. The present invention addresses these needs.
Accordingly, one preferred embodiment of the invention provides a chemical process for preparing a mixed metal oxide powder which employs as a carrier a non-chelating polymer. A specific preferred process includes the steps of forming a substantially homogeneous liquid mixture containing cations of at least two metals and a non-chelating polymer containing partially negatively charged functional groups, wherein the mixture contains an excess of the cations relative to the partially negatively charged functional groups on the polymer; removing the liquid from the solution to form a solid precursor material; and, calcining the solid precursor material to form a mixed metal oxide material. In preferred modes of carrying out this process of the invention, the non-chelating polymer is a polyhydroxyl polymer such as polvinyl alcohol or a glycol such as polyethylene glycol. The polymer can be preformed and added to make the mixture, and/or precursor monomers to the polymer can be added and polymerized to form the polymer in situ. In either event, although it is not intended that the present invention be limited by any theory, it is believed that the non-chelating polymer serves to sterically entrap the metal cations in a manner which facilitates the formation of an atomistically homogeneous, pre-ceramic organometallic. Thus, the presence of a chelating polymer, e.g. one derived from citric acid as in the well-known Pechini method, is unnecessary to achieving quality, mixed oxide powders in accordance with the present invention.
Another preferred embodiment of the present invention provides a process for preparing a mixed metal oxide powder, which includes the step of calcining a gel containing cations of at least two metals and a non-chelating polymer so as to form a mixed metal oxide powder compound.
Another preferred embodiment of the invention provides a process for preparing a powder of a metal oxide containing at least two metals in a specified stoichiometric ratio. The process includes the steps of forming a substantially homogeneous liquid mixture containing cations of at least two metals and a non-chelating polymer that is formed in situ with the cations; removing the liquid from the mixture to form a solid precursor material; and calcining the solid precursor material to form the mixed metal oxide powder.
Still another preferred aspect of the invention provides a process for preparing a powder of a metal oxide containing at least two metals in a fixed stoichiometric ratio. This process involves providing a solution comprising a solvent, cations of at least two metals dissolved in the solvent in a stoichiometric ratio, and a carrier polymer dissolved in the solvent, the carrier polymer having negatively charged functional groups, wherein the ratio of the cationic valences of the cations to the partially negatively charged functional groups is about 4:1 to about 12:1. An organometallic precursor gel is formed from the liquid precursor by removing the solvent. The organometallic precursor gel is calcined to form the powder.
Additional embodiments of the invention provide mixed oxide powders prepared by processes of the invention.
The present invention provides chemical routes to mixed metal oxide powders that achieve exceptionally high yields of quality powders while employing relatively low temperatures. Additional objects and advantages of the invention will be apparent from the description that follows.